This invention relates to adaptive filters in which the amount of filtering varies as a function of a parameter of a signal.
A number of dynamic filters have appeared in the literature. U.S. Pat. No. 3,767,939 to Chamran et al discloses a system for automatically controlling the time constant of an electronic filter in response to the amount and rate of change in the input signal for values greater than a predetermined value. U.S. Pat. No. 3,946,211 to Jenkins represents a similar approach in which the filter time constant is controlled to one of two levels by the amplitude of the input signal being either above or below a preset amplitude limit. Both of these prior art patents then disclose a step-function type of filter constant arrangement, wherein the filter response is at one constant level below a preset input signal level and at another level above the preset input signal level.
A second prior art approach has been to replace the lower step portion of the above filters with a ramp which slopes linearly down from the upper step filter value of some maximum filter value at intersection of the filter function with the ordinate axis (see FIG. 1B). Nevertheless, the second approach suffers from the same shortcomings as the first; that is, undesirable trade-offs between amount of filtering and amount of lag introduced by the filtering.
However, in many applications, the simple filter functions of the prior art are not adequate. In certain systems, the ratio of signal to noise in the system is not a constant and requires sophisticated filtering. Attempts to apply the prior art two-step filter functions met with failure. A new filtering approach was clearly needed.